1. Technical Field of the Invention
The present invention relates generally to an electrophotographic color imaging apparatus which is designed to superimpose two or more single color toner images to form a multicolor composite image on a sheet of transfer paper.
2. Background Art
In recent years, electrophotography using dry toner materials has become used most widely in electrophotographic printing arts, and is put to practical use in copying machines or laser beam printers.
In conventional electrophotographic color printers, an exposure beam for each color is radiated onto a photosensitive medium having a photosensitive layer thereon to form a plurality of electrostatic latent images which are, in turn, toned with different color toners, respectively and then transferred to a sheet of transfer paper to create a multicolor image.
U.S. Pat. No. 4,652,115 and Japanese Patent First Publication No. 63-292156 teach a conventional electrophotographic color printer wherein an intermediate transfer medium is provided between a photosensitive medium and a transfer paper feeding path so that toner images formed on the photosensitive medium are superimposed on the intermediate transfer medium to form a composite image which is, in turn, transferred onto a sheet of transfer paper.
Hereinafter, an example of conventional electrophotographic color printer will be explained with reference to FIGS. 7 to 11.
The shown electrophotographic color printer includes a photosensitive medium 2 provided with an endless resinous belt on which a photosensitive film made of selenium (Se) or organic photoconductive (OPC) materials is formed, an exposure roller 3, a drive roller 4, a tension roller 5 all of which support the photosensitive medium 2, and an electrifier 6 which electrifies the photosensitive medium 2.
The photosensitive medium 2 defines a vertical flat surface between the exposure roller 3 and the drive roller 4 which are fixed on a chassis. The tension roller 5 provides tension to the photosensitive medium 2 outwardly in a direction shown by arrow C. The drive roller 4 moves the photosensitive medium 2 along a given path of travel in a direction shown by arrow A. Disposed around the photosensitive medium 2 are the electrifier 6, an exposure optical system 7, black (K), yellow (Y), magenta (M), and cyanogen (C) toner developing devices 8K, 8Y, 8M, and 8C, an intermediate transfer unit 9, a photosensitive medium cleaning device 10, and a discharging device 11.
The electrifier 6 includes a charging wire 12 made of such as tungsten, a metal shield plate 13, and a grid plate 14. When a high voltage of the order of 4-5 kV is applied to the charging wire 12, it will cause corona discharge to take place on the charging wire 12 so that the photosensitive medium 2 is charged uniformly through the grid plate 14.
An exposure beam 15 is provided with an image signal from a gradient converter which is light intensity-modulated or pulse width-modulated by a laser drive circuit and then radiated from a laser diode to the photosensitive medium 2 to form a single color electrostatic latent image. The developing devices 8K, 8Y, 8M, and 8C store therein different color toners, respectively and include developing roller 34K, 34Y, 34M, and 34C for discharging the color toners. The multicolor developing operations are performed by rotating cams 16K, 16Y, 16M, and 16C in sequence according to color selection signals so that the selected developing device, for example, the developing device 8K is urged into engagement with the photosensitive medium 2, while maintaining the other developing devices 8Y, 8M, and 8C away from the photosensitive medium 2.
In operation, when the cam 16K for the black toner developing device 8K is, as shown in FIG. 8, rotated 180 deg., it will cause the black toner developing device 8K to be shifted toward in a direction shown by arrow B so that the developing roller 34K engages the photosensitive medium 2. A spring 40 is provided to bias the developing device 8K in a direction indicated by arrow G into constant engagement with the cam 16K. A spring force of the spring 40 is set to a relatively lower value because increasing the spring force causes a resistance to the movement of the developing device 8K by the cam 16K to be increased, thus requiring greater driving force for the cam 16K. One end of the spring 40 is attached to the chassis of the printer, while the other end thereof is connected to the developing device 8K through a stopper 35. A sensor 36 is provided to monitor position of the stopper 35.
The developing roller 34K is rotated by rotating a developing roller gear 37 mounted on an end thereof through a clutch gear 38 with a drive motor (not shown). The clutch gear 38 also provides torque to a toner hopper 100K, as shown in FIG. 7, to carry a toner in the developing device 8K to the developing roller 34K.
Referring back to FIG. 7, the Intermediate transfer unit 9 includes an intermediate transfer belt 17 provided with an endless loop made of a conductive resin, an intermediate transfer belt drive roller 18 supporting the transfer belt 17, an intermediate transfer belt transfer roller 19, an intermediate transfer belt tension roller 20, and an intermediate transfer roller 21. The intermediate transfer belt tension roller 20 is urged outwardly in a direction shown by arrow D. The intermediate transfer roller 21 is arranged to hold the photosensitive medium 17 together with the transfer belt 17. The drive roller 18 is driven by the drive motor to move the transfer belt 17 in a direction shown by arrow E. A reference position sensor 22 is arranged to detect an intermediate transfer belt reference mark provided with such as a slit formed in an end of the intermediate transfer belt 17 to determine a reference position thereof. An intermediate transfer belt cleaning device 23 is disposed adjacent the drive roller 18 to remove a superfluous toner left on the transfer belt 17, and is designed to remain separate from the transfer belt 17 during formation of a composite image of each color on the transfer belt 17 and engages the transfer belt 17 only when cleaning it. A transfer paper cassette 24 stores therein a stack of sheets of transfer paper 25. A paper feeding roller 26 carries a sheet of transfer paper 25 from the transfer paper cassette 24 toward a paper transport path 27. A register roller 28 engages a compliance roller 29 and serves to stop transport of the transfer paper 25 temporarily to bring it into registration with the composite image formed on the intermediate transfer belt 17. A transfer roller 30 serves to transfer the composite image on the transfer belt 17 onto the transfer paper 25 and is designed to rotate only during transfer in contact with the transfer belt 17. A fixing device 31 includes a heat roller 32 having therein a heat source and a press roller 33 which hold therebetween the transfer paper 25 to fix the composite image on the transfer paper under heat and pressure so that a desired multicolor image is formed.
An image-transferring operation will be described below.
The photosensitive medium 2 and the intermediate transfer belt 17 are driven by the drive motor respectively at a constant speed, matching a peripheral speed with each other. The reference mark of the intermediate transfer belt 17 is detected by the reference position sensor 22 to determine an image-forming area on the transfer belt 17.
In the above condition, a high-voltage on the order of -4 kV to -5 kV is applied to the charging wire 12 of the electrifier 6 to produce corona discharge so that a surface of the photosensitive medium 2 is charged uniformly at a voltage on the order of -700 kV. Subsequently, the photosensitive medium 2 is rotated in the direction A, and the exposure beam 15 which is provided with a laser beam corresponding to one (e.g., black) of multiple colors, is radiated onto the uniformly-charged photosensitive medium 2. On a portion of the photosensitive medium 2 which is exposed to the exposure beams, a charge disappears so that a latent image is formed. The timing where the formation of the latent image is initiated is determined by a signal from the reference position sensor 22.
The black toner developing device 8K is brought into contact with the photosensitive medium 2 urged in the direction B, as shown in FIG. 8, according to rotation of the cam 16K after a period of time following detection of the reference mark on the intermediate transfer belt 17 by the reference position sensor 22. Just before this contact, a negative voltage of the order of -300 kV is applied to the developing roller 34K disposing a toner on its surface. This is because the provision of the negative voltage on the toner allows only a portion of the photosensitive medium 2 from which a charge disappears due to the radiation of the exposure beam 15 to be deposited with the toner. After completion of the developing operation, the developing device 8K is shifted away from the photosensitive medium 2 in the direction G according to 180 deg. of rotation of the cam 16K. The toner image formed on the photosensitive medium 2 by the developing device 8K is transferred to the intermediate transfer belt 17 by applying a high voltage of the order of +800 kV to the intermediate transfer roller 21 for every color.
The remainder of the toner which has not been transferred from the photosensitive medium 2 to the intermediate transfer belt 17, is removed by a cleaning blade 10a provided in the photosensitive medium cleaning device 10, and a charge on the photosensitive medium 2 is removed by the discharging device 11.
Subsequently, upon selection of the cyanogen (C), the cam 16C is rotated to press the cyanogen toner developing device 8C against the photosensitive medium 2 so that the photosensitive medium 2 is toned with a cyanogen toner. In the case of an electrophotographic color printer using four color toners, the above mentioned developing operation is sequentially repeated four times so that black, yellow, magenta, and cyanogen toner images are superimposed to form a composite toner image on the intermediate transfer belt 17.
The transfer paper 25 is carried from the transfer paper cassette 24 along a transfer paper feeding path 27 and then is stopped at the register roller 28 for controlling operational timing where a composite image is transferred. After the transfer roller 30 is brought into contact with the intermediate transfer belt 17 and a high-voltage of the order of +1000 V is applied to the transfer roller 30, the composite toner image formed on the intermediate transfer belt 17 is timed by the register roller 28 to be transferred under pressure to the transfer paper being carried at a time.
Any residual toner which may be left on the intermediate transfer belt 17 without being transferred to the transfer paper 25 is removed by the transfer belt cleaning device 23. The composite image-transferred transfer paper 25 is fed to the fixing device 31 so that the composite image is fixed under heat of the heat roller 32 and pressure of the press roller 33 to form a multicolor image.
The transfer belt cleaning device 23 remains separate from the intermediate transfer belt 17 until the composite image has been transferred to the transfer paper 25.
With the above image-transferring operation, the multicolor image is recorded on a sheet of transfer paper completely.
Hereinbelow, engagement and disengagement of the developing device 8K with and from the photosensitive medium 2 will be discussed with reference to FIGS. 9 to 11.
The developing device 8K in a standby state, as shown in FIG. 10(a), is responsive to rotation of the cam 16K to move toward the photosensitive medium 2 at a time P, as shown in FIG. 9, prior to a time N at which a leading edge of the image transferred onto the photosensitive medium 2 reaches the developing device 8K. The developing roller 34K, as shown in FIG. 10(b), then engages the photosensitive medium 2. The cam 16K stops rotating upon the sensor 36 detecting a slit formed on the developing device 8K at a time Q following a time L at which the developing roller 34K engages the photosensitive medium 2 after which the developing roller 34K, as shown in FIG. 10(c), begins to rotate at a time T.
The developing roller 34K stops rotating in engagement with the photosensitive medium 2, as shown in FIG. 11(b), at a time U after a time O at which a trailing edge of the image formed on the photosensitive medium 2 passes over the developing roller 34K. Subsequently, the cam 16K begins to rotate at a time R, the developing roller 34K is, as shown in FIG. 11(c), disengaged from the photosensitive medium 2, the sensor 36 detects the absence of the slit provided on the developing device 8K in a detecting range, and then the cam 16K stops rotating, a sequence of disengagement operations thereby terminating.
The above described prior art arrangements, however, encounters the following drawbacks. At a time when the developing roller 34K engages the photosensitive medium 2, the developing roller 34K does not yet start to rotate, thus causing a toner layer formed on a surface of the developing roller 34 to be stripped by a surface of the traveling photosensitive medium 2. This can lead to direct contact of the developing roller 34 with the photosensitive medium 2, causing friction to create between the developing roller 34K and the photosensitive medium 2. This results in a change in traveling speed of the photosensitive medium 2. Thus, in the event that a latent image is being formed by the radiation of the exposure beam 15, it may be disturbed, resulting in quality of an output image being degraded.
To eliminate the above problem, if the developing roller 34K is made to rotate before engagement with the photosensitive medium 2, that is, at a location distant from the photosensitive medium 2, another problem is raised in that in the case where the clutch gear 38 is located above the developing roller gear 37, as shown in the drawings, the torque of the clutch gear 38 which essentially serves to rotate the developing roller gear 37 also acts on the developing device 8K through the developing roller gear 37 and its support shaft to urge it toward the photosensitive medium 2 against a biasing force of the spring 40, thereby causing the developing roller 34K to hit the photosensitive medium 2 hard. This can lead to vibrations, resulting in latent images being disturbed with the degradation of quality of output images.
In addition, even when the developing roller 34K is made to disengage from the photosensitive medium while being maintained rotating, In the case where the clutch gear 28 is arranged above the developing roller gear 37, the developing device 8K is maintained urged toward the photosensitive medium 2 during a time when the developing roller 34K is driven to rotate. Upon stopping the developing roller 34k from rotating, the developing device 8K is suddenly shifted toward the cam 16 due to the biasing force of the spring 40, causing vibrations to occur which may affect the formation of the latent images.
Further, in the case where the clutch gear 38 is arranged below the developing roller gear 37, the torque of the clutch gear 38 also acts to urge the developing device 8K in a direction opposite the photosensitive medium 2. This produces a resistance to the movement of the developing device 8K by the cam 16K, thus requiring more torque to drive the cam 16K for displacing the developing device 8K toward the photosensitive medium 2. Accordingly, a large drive motor is needed, resulting in the system becoming bulky with increased power consumption.